The use of watthour meters to determine the amount of electrical energy consumed by a customer is well known and is in widespread practice. The manufacturers of watthour meters have the continual problem of building a watthour meter and then calibrating it, that is, determining how different any given production watthour meter might be from a standard watthour meter. Such calibrations involve either adjusting the production watthour meters in the course of testing the same in order to have such production items approach the performance of a reference watthour meter, or, alternatively, providing a correction factor to be used with the read out of such production watthour meters.
In recent times, with the advent of electronic counters and electronic computers, it has become the practice to use a reference watthour meter which has been calibrated against a government standard watthour meter as a source of pulses. Usually the disk of such a standard watthour meter has had its edge serrated and such a disk is passed under a light source so that the beam of light therefrom is interrupted in accordance with the serrated edge. The interrupted light is transmitted to a light sensitive device and in accordance with the interruptions, there are electrical pulses generated, i.e., corresponding to the interruptions. In an alternative operation, holes are drilled in the disk and they serve the same purpose as the serrated edge, i.e. to provide interruptions to a light beam being passed through the holes in the disk. In such arrangements the pulses are electrically counted. The pulses are counted during a period which is commenced with a start signal, from the disk of the production meter under test, and stop being counted, in accordance with a stop signal from the disk of the meter under test. It follows that if the start and stop signal coincide with one revolution of the disk of the watthour meter under test, then the pulses counted should represent the number of pulses available during one complete excursion of the disk of the watthour meter under test. The number of the pulses so counted is compared with a desired number of pulses representing the number of pulses that would be available for a complete excursion of the disk of the standard watthour meter and any difference represents an inaccuracy of the production watthour meter (as compared with the standard watthour meter). Such an inaccuracy is the basis of an adjustment or of the provision of a factor to be used in the read out. Such a device is described in U.S. Pat. No. 2,938,165 issued to W. A. Greig.
Major problems exist with such techniques, that is the technique of using a standard watthour meter as the source of pulses. The degree of accuracy, that is the degree to which the difference between the standard watthour meter and the production watthour meter can be measured, is limited by the pulses which can be provided from the disk of the standard, or reference, watthour meter. In addition there are further limitations, such as the fact that an electromechanical watthour standard is insensitive to waveform distortion, and this is a source of errors. The further limitations also reside in the fact that an electromechanical watthour standard is sensitive to changes in temperature and to vibrations.